Project Summary Preventing high intensity drinking episodes (HID; consuming 8+ standard drinks for women and 10+ for men) among emerging adults (EAs) is an important public health concern; HID increases risk for both short (e.g., injury) and long-term consequences of alcohol use. Developmentally, EAs report the highest prevalence of alcohol use and misuse. Approximately 11% of EAs report past 2-week HID, 1/3 of those who binge drink (5+ drinks) report HID in the past 2 weeks, and 1 in 7 high school seniors follow a sustained HID pattern through emerging adulthood. HID increases risk for blackouts, potentially leading to further health consequences such as injury. Although there are promising interventions that reduce alcohol consumption among EAs, intervention effects are typically modest in size without sustained effects over time, and interventions to date have not addressed HID specifically, which may be less likely to change as a result of previous interventions. Engaging with social media (SM) is now a daily norm for EAs with regular use of private mobile-messaging apps like Snapchat among ~80% of EAs. Users under age 25 spend an M=40min/day on Snapchat. These high levels of engagement with SM content provide a dynamic platform to engage EAs in interventions providing access to peer-like health coaches to address HID in their daily lives closer to in real-time than traditional interventions. The specific aims of the proposed study are to: 1) iteratively develop and focus test a peer-facilitated 8-week SM-based intervention for HID among EAs, and 2) test the preliminary efficacy of the intervention versus a control condition on HID and alcohol consequences. We will iteratively develop a peer-facilitated private SM messaging intervention delivered via Snapchat. As in prior work, we will initially develop engaging, user- generated content via a participatory approach with EAs and national crowdsourcing, followed by iterative focus testing with nationally recruited EA participants with HID. The intervention will leverage peers as health coaches to promote rapport and connection (which is scalable given access to student trainees in health fields) and will focus on the ?Why, How? model of motivational interviewing with tailoring of content addressing ?why? change (e.g., elicit reasons for change, review of negative consequences, normative feedback on HID, etc.) and ?how? change (e.g., elicit benefits of protective strategies, addressing expectancies and motives, use of cognitive behavioral coping strategies for motives, goal setting, etc.). We will test the developed intervention in a pilot randomized controlled trial versus a control condition among 100 18-25 year-old EAs with past-month HID. Outcomes will be measured at a 2-month post-test and 4-month follow-up (post-baseline). The development of an adaptable, scalable and efficacious intervention tailored for HID among EAs is a critical step in public health efforts to high-risk drinking and related consequences. This study could have significant impact by altering HID trajectories among EAs, preventing health and social consequences and the development of alcohol use disorders, as well as informing future SM interventions for behavioral health.